Load control device having an overcurrent protection circuit

ABSTRACT

A load control device for controlling power delivered from an alternating-current power source to an electrical load may comprise a controllably conductive device, a control circuit, and an overcurrent protection circuit that is configured to be disabled when the controllably conductive device is non-conductive. The control circuit may be configured to control the controllably conductive device to be non-conductive at the beginning of each half-cycle of the AC power source and to render the controllably conductive device conductive at a firing time during each half-cycle (e.g., using a forward phase-control dimming technique). The overcurrent protection circuit may be configured to render the controllably conductive device non-conductive in the event of an overcurrent condition in the controllably conductive device. The overcurrent protection circuit may be disabled when the controllably conductive device is non-conductive and enabled after the firing time when the controllably conductive device is rendered conductive during each half-cycle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 16/515,411, filed on Jul. 18, 2019, which is acontinuation of U.S. Non-Provisional patent application Ser. No.16/003,909, filed on Jun. 8, 2018, now U.S. Pat. No. 10,362,656, issuedon Jul. 23, 2019, which claims priority to U.S. Provisional PatentApplication No. 62/517,484, filed Jun. 9, 2017, the entire disclosuresof which are incorporated by reference herein.

BACKGROUND

Prior art load control devices, such as dimmer switches, may be coupledin series electrical connection between an alternating-current (AC)power source and a lighting load for controlling the amount of powerdelivered from the AC power source to the lighting load. A standarddimmer switch may typically comprise a bidirectional semiconductorswitch, e.g., a thyristor (e.g., such as a triac) or two field-effecttransistors (FETs) in anti-series connection. The bidirectionalsemiconductor switch may be coupled in series between the AC powersource and the load and is controlled to be conductive andnon-conductive for portions of a half cycle of the AC power source tothus control the amount of power delivered to the electrical load.Generally, dimmer switches may use either a forward phase-controldimming technique or a reverse phase-control dimming technique in orderto control when the bidirectional semiconductor switch is renderedconductive and non-conductive to thus control the power delivered to theload. The dimmer switch may comprise a toggle actuator for turning thelighting load on and off and an intensity adjustment actuator foradjusting the intensity of the lighting load. Examples of prior artdimmer switches are described in greater detail is commonly-assignedU.S. Pat. No. 5,248,919, issued Sep. 29, 1993, entitled LIGHTING CONTROLDEVICE; and U.S. Pat. No. 6,969,959, issued Nov. 29, 2005, entitledELECTRONIC CONTROL SYSTEMS AND METHODS; the entire disclosures of whichare incorporated by reference herein.

In order to save energy, high-efficiency lighting loads, such as, forexample, light-emitting diode (LED) light sources, are being used inplace of or as replacements for conventional incandescent or halogenlamps. High-efficiency light sources typically consume less power andprovide longer operational lives as compared to incandescent and halogenlamps. In order to illuminate properly, a load regulation circuit (e.g.,such as an electronic dimming ballast or an LED driver) may be coupledbetween the AC power source and the respective high-efficiency lightsource (e.g., the compact fluorescent lamp or the LED light source) forregulating the power supplied to the high-efficiency light source. Somehigh-efficiency lighting loads may be integrally housed with the loadregulation circuit in a single enclosure. Such an enclosure may have ascrew-in base that allows for mechanical attachment to standard Edisonsockets and provide electrical connections to the neutral side of the ACpower source and either the hot side of the AC power source or thedimmed-hot terminal of the dimmer switch (e.g., for receipt of thephase-control voltage).

A dimmer switch for controlling a high-efficiency light source may becoupled in series between the AC power source and the load regulationcircuit for the high-efficiency light source. The load regulationcircuit may control the intensity of the high-efficiency light source tothe desired intensity in response to the conduction time of thebidirectional semiconductor switch of the dimmer switch. The loadregulation circuits for the high-efficiency light sources may have highinput impedances or input impedances that vary in magnitude throughout ahalf cycle. When a prior-art forward phase-control dimmer switch iscoupled between the AC power source and the load regulation circuit forthe high-efficiency light source, the load regulation circuit may not beable to conduct enough current to exceed the rated latching and/orholding currents of the thyristor.

SUMMARY

As described herein, a load control device for controlling powerdelivered from an alternating-current (AC) power source to an electricalload may comprise a controllably conductive device, a control circuit,and an overcurrent protection circuit that is configured to be disabledwhen the controllably conductive device is non-conductive. Thecontrollably conductive device may be adapted to be coupled between theAC power source and the electrical load for controlling the powerdelivered to the electrical load. For example, the controllablyconductive device may comprise two field-effect transistors (FETs)coupled in anti-series connection. The control circuit may be configuredto control the controllably conductive device using a forwardphase-control dimming technique. The control circuit may control thecontrollably conductive device to be non-conductive at the beginning ofeach half-cycle of the AC power source and to render the controllablyconductive device conductive at a firing time during each half-cycle.The overcurrent protection circuit may be coupled to the controllablyconductive device and may render the controllably conductive devicenon-conductive in the event of an overcurrent condition in thecontrollably conductive device. The overcurrent protection circuit maybe disabled when the controllably conductive device is non-conductiveand enabled after the firing time when the controllably conductivedevice is rendered conductive during each half-cycle.

In addition, a method of controlling power delivered from analternating-current (AC) power source to an electrical load is alsodisclosed herein. The method may comprise: (1) controlling acontrollably conductive device using a forward phase-control techniqueto conduct a load current through the electrical load to control thepower delivered to the electrical load; (2) controlling the controllablyconductive device to be non-conductive at the beginning of eachhalf-cycle of the AC power source; (3) disabling an overcurrentprotection circuit when the controllably conductive device isnon-conductive during each half-cycle, the overcurrent protectioncircuit coupled to the controllably conductive device and responsive tothe magnitude of the load current; (4) rendering the controllablyconductive device conductive at a firing time during each half-cycle;and (5) enabling the overcurrent protection circuit after the firingtime when the controllably conductive device is rendered conductiveduring each half-cycle to allow the overcurrent protection circuit torender the controllably conductive device non-conductive in the event ofan overcurrent condition in the controllably conductive device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an example load control device(e.g., a dimmer switch) for controlling the amount of power delivered toan electrical load, such as, a lighting load.

FIG. 2 is a simplified schematic diagram of another example load controldevice showing an overcurrent protection circuit and an overridecircuit.

FIG. 3 shows simplified waveforms that illustrate the operation of theload control device of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of an example load control device100 (e.g., a dimmer switch) for controlling the amount of powerdelivered to an electrical load, such as, a lighting load 102. The loadcontrol device 100 may have a hot terminal H coupled to analternating-current (AC) power source 104 for receiving an AC mains linevoltage V_(AC), and a dimmed-hot terminal DH coupled to the lightingload 102.

The load control device 100 may comprise a controllably conductivedevice 110, such as two field-effect transistors (FETs) Q112, Q114 thatmay be coupled in anti-series connection between the hot terminal andthe dimmed-hot terminal DH. The junction of the FETs may be coupled tocircuit common. The load control device 100 may comprise a controlcircuit 115, e.g., a digital control circuit, for controlling the FETsQ112, Q114 to conduct a load current I_(LOAD) through the lighting load102. The control circuit 115 may include one or more of a processor(e.g., a microprocessor), a microcontroller, a programmable logic device(PLD), a field programmable gate array (FPGA), an application specificintegrated circuit (ASIC), or any suitable controller or processingdevice. The control circuit 115 may generate first and second drivesignals V_(DR1), V_(DR2) that may be coupled to the gates of therespective FETs Q112, Q114 via first and second gate drive circuits 116,118, respectively, for generating gate voltages V_(G1), V_(G2) at thegates of the FETs. For example, the first and second gate voltagesV_(G1), V_(G2) may be the inverse of the respective drive signalsV_(DR1), V_(DR2). When the controllably conductive device 110 isrendered conductive during the positive half-cycles of the AC powersource 104, the load current I_(LOAD) may be conducted through thedrain-source channel of the first FET Q112 and the body diode of thesecond FET Q114. When the controllably conductive device 110 is renderedconductive during the negative half-cycles of the AC power source 104,the load current I_(LOAD) may be conducted through the drain-sourcechannel of the second FET Q114 and the body diode of the first FET Q112.

The control circuit 115 may receive a zero-cross control signal V_(ZC)representative of the zero-crossing points of the AC main line voltageof the AC power source 104 from a zero-cross detect circuit 120. Thecontrol circuit 115 may be configured to render the FETs Q112, Q114conductive and/or non-conductive at predetermined times (e.g., at afiring time or firing angle) relative to the zero-crossing points of theAC waveform to generate a phase-control voltage V_(PC) using aphase-control dimming technique (e.g., a forward phase-control dimmingtechnique and/or a reverse phase-control dimming technique). Examples ofdimmers are described in greater detail in commonly-assigned U.S. Pat.No. 7,242,150, issued Jul. 10, 2007, entitled DIMMER HAVING A POWERSUPPLY MONITORING CIRCUIT; U.S. Pat. No. 7,546,473, issued Jun. 9, 2009,entitled DIMMER HAVING A MICROPROCESSOR-CONTROLLED POWER SUPPLY; andU.S. Pat. No. 8,664,881, issued Mar. 4, 2014, entitled TWO-WIRE DIMMERSWITCH FOR LOW-POWER LOADS, the entire disclosures of which areincorporated by reference herein.

The load control device 100 may include a power supply 122. The powersupply 122 may generate a direct-current (DC) supply voltage V_(CC) forpowering the control circuit 115 and the other low-voltage circuitry ofthe load control device 100. The power supply 100 may be coupled inparallel with the series combination of the FETs Q112, Q114. The powersupply 122 may be configured to conduct a charging current through thelighting load 102 to generate the DC supply voltage V_(CC).

The load control device 100 may further comprise an overcurrentprotection circuit 130 that may be coupled across the series combinationof the FETs Q112, Q114 for receiving the voltage developed across theFETs. The voltage developed across the series combination of the FETsQ112, Q114 may be a function of the magnitude of the load currentI_(LOAD) and an on resistance R_(DS-ON) of the conducting FET as well asthe forward voltage drop of the body diode of the non-conducting FET.Thus, the voltage developed across the controllably conductive device110 (e.g., across the series combination of the FETs Q112, Q114) may berepresentative of the magnitude of the load current I_(LOAD). Theovercurrent protection circuit 130 may be responsive to the magnitude ofthe load current I_(LOAD) (e.g., responsive to the magnitude of thevoltage developed across the controllably conductive device 110, whichmay indicate the magnitude of the load current I_(LOAD)). Theovercurrent protection circuit 130 may be electrically coupled to thegates of the FETs Q112, Q114 for controlling the FETs Q112, Q114 in theevent of an overcurrent condition. For example, the overcurrentprotection circuit 130 may be configured to control the magnitude of thegate voltages V_(G1), V_(G2) to approximately zero volts by shortinggates of the respective FET Q112, Q114 to circuit common.

The control circuit 115 may be coupled to the overcurrent protectioncircuit 130 for enabling and disabling the overcurrent protectioncircuit 130. For example, the control circuit 115 may generate an enablecontrol signal V_(ENABLE) for enabling and disabling the overcurrentprotection circuit 130. When the control circuit 115 is controlling theFETs Q112, Q114 using the forward phase-control dimming technique, thecontrol circuit 115 may be configured to disable the overcurrentprotection circuit 130 while the controllably conductive device 110 isnon-conductive during each half-cycle of the AC power source 104 (e.g.,when one of the FETs Q112, Q114 is rendered non-conductive to block theflow of the load current I_(LOAD)). The overcurrent protection circuit130 may be disabled while the controllably conductive device 110 isnon-conductive during each half-cycle to prevent the overcurrentprotection circuit 130 from tripping when the controllably conductivedevice 110 is rendered conductive during each half-cycle (e.g., at thefiring time or firing angle). After control circuit 115 control one ofthe FETs Q112, Q114 to render the controllably conductive device 100conductive, the magnitude of the phase-control voltage V_(PC) maytransition from approximately zero volts to approximately the magnitudeof the AC mains line voltage V_(AC) over a switching time period (e.g.,a rise time period and/or a turn-on time period). In addition, thecontrol circuit 115 may be configured to delay enabling the overcurrentprotection circuit 130 for a delay time period after the time at whichone of the FETs Q112, Q114 is controlled to render the controllablyconductive device 110 conductive during each half-cycle, for example, toallow the FET to become fully conductive during the switching timeperiod.

While the two FETs Q112, Q114 are shown in FIG. 1, the two FETs may bereplaced by a single FET in a full-wave rectifier bridge. In such animplementation, the control circuit 115 may generate a single drivevoltage for producing a single gate voltage at the gate of the FET inthe bridge. The overcurrent protection circuit 130 may be coupled acrossthe FET in the bridge and would be responsive to the voltage across theFET and thus the current conducted through the FET. The overcurrentprotection circuit 130 may be configured to remove the gate voltage fromthe gate of the FET in the event of an overcurrent condition. Thecontrol circuit 115 would be configured to disable the overcurrentprotection circuit when the FET is non-conductive during each half-cyclein a similar manner as described above.

FIG. 2 is a simplified schematic diagram of another example load controldevice 200 (e.g., the load control device 100 shown in FIG. 1) forcontrolling the amount of power delivered to an electrical load, such asa lighting load (e.g., the lighting load 102). FIG. 3 shows simplifiedwaveforms that illustrate the operation of the load control device 200.The load control device 200 may comprise a controllably conductivedevice 210, for example, including two FETs Q212, Q214 coupled inanti-series connection between a hot terminal H (e.g., that may becoupled to an AC power source) and a dimmed-hot terminal DH (e.g., thatmay be coupled to the lighting load). The junction of the FETs Q212,Q214 may be coupled to circuit common. The load control device 200 maycomprise a control circuit 215 (e.g., a digital control circuit)configured to control the FETs Q212, Q214 using a forward phase-controldimming technique to generate a phase-control voltage V_(PC) to beprovided to the lighting load (e.g., a forward phase-control voltage asshown in FIG. 3) and conduct a load current I_(LOAD) through thelighting load. The control circuit 215 may include one or more of aprocessor (e.g., a microprocessor), a microcontroller, a programmablelogic device (PLD), a field programmable gate array (FPGA), anapplication specific integrated circuit (ASIC), or any suitablecontroller or processing device. The control circuit 215 may be poweredfrom a first supply voltage V_(CC) (e.g., approximately 3.3 volts or 5volts), which may be generated by a power supply of the load controldevice 200 (e.g., the power supply 122 shown in FIG. 1).

The control circuit 215 may generate first and second drive signalsV_(DR1), V_(DR2) for controlling the magnitude of the phase-controlvoltage to be approximately equal to zero volts for a non-conductiontime period T_(NC) at the beginning of each half-cycle and approximatelyequal to the magnitude of the AC line voltage for a conduction timeperiod T_(CON) at the end of each half-cycle. The control circuit 215may be configured to drive the first drive signal V_(DR1) high (e.g.,towards the first supply voltage V_(CC)) to render the first FET Q212non-conductive for the non-conductive time period T_(NC) during thepositive half-cycles, and to drive the second drive signal V_(DR2) high(e.g., towards the first supply voltage V_(CC)) to render the first FETQ212 non-conductive for the non-conductive time period T_(NC) during thenegative half-cycles (e.g., as shown in FIG. 3). The first and seconddrive signals V_(DR1), V_(DR2) may be coupled to the gates of therespective FETs Q212, Q214 via first and second gate drive circuits 216,218, respectively, for generating gate voltages V_(G1), V_(G2). Thefirst and second gate drive circuits 216, 218 may pull the gates of therespective FETs Q212, Q214 up towards a second supply voltage V_(CC2)(e.g., approximately 12 volts) when the respective drive signalsV_(DR1), V_(DR2) is driven low towards circuit common as shown in FIG.3). The FETs Q212, Q214 may be rendered conductive when the gatevoltages V_(G1), V_(G2) are driven above rated gate threshold voltagesof the FETs.

The load control device 200 may further comprise an overcurrentprotection circuit 230 that may be coupled across the series combinationof the FETs Q212, Q214 for receiving the voltage developed across theFETs. The overcurrent protection circuit 230 may comprise two resistorsR231, R232 that may be coupled across the series combination of the FETsQ212, Q214. The junction of the resistors R231, R232 may be coupled tocircuit common through a sense resistor R234, such that the seriescombination of the first resistor R231 and the sense resistor R234 maybe coupled in parallel with the drain-source junction of the first FETQ212 and the series combination of the second resistor R232 and thesense resistor R234 may be coupled in parallel with the drain-sourcejunction of the second FET Q214. The sense resistor R234 may be coupledacross the base-emitter junction of a transistor Q236, e.g., an NPNbipolar junction transistor (BJT). The collector of the transistor Q236may be coupled to the gate of the first FET Q212 through a diode D238and to the gate of the second FET Q214 through a diode D239.

In the event of an overcurrent condition (e.g., if the magnitude of theload current I_(LOAD) exceeds an overcurrent threshold) while thecontrollably conductive device 210 is conductive, the overcurrentprotection circuit 230 may render the FETs Q212, Q214 non-conductive.For example, the overcurrent protection circuit 230 may render thecontrollably conductive device 210 non-conductive, for example, bycontrolling the magnitude of the gate voltages V_(G1), V_(G2) towardscircuit common (e.g., to a voltage less than the rated gate thresholdvoltages of the FETs) to render both of the FETs non-conductive. Theovercurrent threshold may be set such that the overcurrent protectioncircuit 230 does not render the FETs Q212, Q214 non-conductive duringnormal operation (e.g., even during the conduction of an inrush currentwhen the lighting load is first turned on). For example, the overcurrentthreshold may be set such that the overcurrent protection circuit 230renders the FETs Q212, Q214 non-conductive if the magnitude of the loadcurrent I_(LOAD) exceeds approximately 70 amps. In addition, theovercurrent protection circuit 230 may generate an overcurrent feedbacksignal, which may indicate an overcurrent condition and may be receivedby the control circuit 215, and the control circuit may be configured tocontrol gate voltages V_(G1), V_(G2) to render the FETs Q212, Q214non-conductive in response to the overcurrent feedback signal.

When the first FET Q212 is rendered conductive, the overcurrentprotection circuit 230 may render both FETs non-conductive Q212, Q214 ifthe magnitude of the load current I_(LOAD) conducted through the firstFET Q212 exceeds the overcurrent threshold. The voltage developed acrossthe series combination of the first resistor R231 and the sense resistorR234 may be a function of the magnitude of the load current I_(LOAD) andan on resistance R_(DS-ON1) of the first FET Q212 when the drain-sourcechannel of the first FET Q212 is conducting the load current I_(LOAD).When the magnitude of the load current I_(LOAD) increases during anovercurrent condition, the voltage developed across the first FET Q212due to the on resistance R_(DS-ON1) may increase significantly. Becausethe body diode of the second FET Q214 is coupled across the secondresistor R232 and the sense resistor R234, the voltage developed acrossthe second FET Q214 during the overcurrent condition does notappreciably affect the voltage developed across the sense resistor R236.When the magnitude of the load current I_(LOAD) exceeds the overcurrentthreshold, the voltage across the sense resistor R234 may exceed a ratedbase-emitter voltage of the transistor Q236, which may render thetransistor Q236 conductive. Accordingly, the gate of the first FET Q212may be pulled down towards circuit common through the first diode D238and the transistor Q236, thus rendering the first FET Q212non-conductive. Since the first FET Q212 is non-conductive, the voltagedeveloped across the FETs Q212, Q214 may be approximately equal to theAC mains line voltage V_(AC), which may maintain the transistor Q236conductive and the first FET Q212 non-conductive (e.g., until themagnitude of the AC mains line voltage V_(AC) drops to zero volts at thenext zero-crossing).

The overcurrent protection circuit 230 may operate in a similar mannerin response to an overcurrent condition in the second FET Q214. Thevoltage developed across the series combination of the second resistorR232 and the sense resistor R234 may be a function of the magnitude ofthe load current I_(LOAD) and an on resistance R_(DS-ON2) of the secondFET Q214 when the drain-source channel of the second FET Q214 isconducting the load current I_(LOAD). When the magnitude of the loadcurrent I_(LOAD) exceeds the overcurrent threshold, the voltagedeveloped across the second FET Q214 due to the on resistance R_(DS-ON2)may increase significantly, which may cause the voltage across the senseresistor R234 to exceed the rated base-emitter voltage of the transistorQ236 and cause the transistor Q236 to be rendered conductive. The gateof the second FET Q214 may be pulled down towards circuit common throughthe second diode D239 and the transistor Q236, thus rendering the secondFET Q214 non-conductive. Since the second FET Q214 is non-conductive,the voltage developed across the FETs Q212, Q214 may be approximatelyequal to the AC mains line voltage V_(AC), which may maintain thetransistor Q236 conductive and the second FET Q214 non-conductive (e.g.,until the magnitude of the AC mains line voltage V_(AC) drops to zerovolts at the next zero-crossing).

The control circuit 215 may be coupled to the overcurrent protectioncircuit 230 through an override circuit 240 for enabling and disablingthe overcurrent protection circuit 230. The override circuit 240 mayreceive the first and second drive signals V_(DR1), V_(DR2) and maygenerate an enable control signal V_(ENABLE) for enabling and disablingthe overcurrent protection circuit 230. The override circuit 240 maycomprise two diodes D241, D242 having anodes coupled to receive thefirst and second drive signals V_(DR1), V_(DR2), respectively, andcathodes coupled together. The junction of the diodes D241, D242 may becoupled to a base of a transistor Q244 (e.g., an NPN bipolar junctiontransistor) through a resistor-capacitor (RC) circuit having a resistorR246 and a capacitor R248. The enable control signal V_(ENABLE) may begenerated at the collector of the transistor Q244, which may be coupledto the base of the transistor Q236 of the overcurrent protection circuit230. In addition, the control circuit 215 may generate the enablecontrol signal V_(ENABLE) (e.g., at an output pin), such that theoverride circuit 240 may not be required.

When the first drive signal V_(DR1) or the second drive signal V_(DR2)is driven high towards the supply voltage V_(CC), the capacitor C248 maycharge through the respective diode D241, D242 and the resistor R246.When the voltage across the capacitor C248 exceeds the ratedbase-emitter voltage of the transistor Q244, the transistor may berendered conductive, thus pulling the enable control signal V_(ENABLE)down towards circuit common. When the enable control signal V_(ENABLE)is low, the transistor Q236 of the overcurrent protection circuit 230 isprevented from being rendered conductive, thus disabling the overcurrentprotection circuit. Since the first and second drive signals V_(DR1),V_(DR2) are driven high to render the respective FET Q212, Q214non-conductive, the overcurrent protection circuit 230 is disabled whenthe FETs Q212, Q214 are non-conductive.

When one of the first and second drive signals V_(DR1), V_(DR2) isdriven low to render the respective FET Q212, Q214 conductive, theovercurrent protection circuit 230 may be enabled after a first delayperiod T_(DELAY1) from when the respective drive signal is driven low(e.g., as shown in FIG. 3). For example, the RC circuit of the overridecircuit 240 may provide the first delay period T_(DELAY1) (e.g., thetime required for the capacitor C248 to discharge to a point where thevoltage across the base-emitter junction of the transistor Q244 dropsbelow the rated base-emitter voltage). The first delay period T_(DELAY1)may be, for example, approximately 60 microseconds, which may be longerthan a switching time period of the FETs Q212, Q214. Similarly, theovercurrent protection circuit 230 may be disabled after a second delayperiod T_(DELAY2) (e.g., approximately 60 microseconds) from when one ofthe first and second drive signals V_(DR1), V_(DR2) is driven high torender the respective FET Q212, Q214 non-conductive.

If the control circuit 215 were to leave the overcurrent protectioncircuit 230 enabled when the controllably conductive device 210 isnon-conductive (e.g., when one or both of the FETs Q212, Q214 arenon-conductive) at the beginning of each half-cycle, the voltagedeveloped across the controllably conductive device may be approximatelyequal to the AC mains line voltage V_(AC), which may cause theovercurrent protection circuit 230 to pull the gate voltages V_(G1),V_(G2) at the gates of the respective FETs Q212, Q214 down towardcircuit common. As a result, the control circuit 215 would not be ableto drive the gate voltages V_(G1), V_(G2) above the rated gate thresholdvoltages of the FETs Q212, Q214, and thus would not be able to renderthe FETs Q212, Q214 conductive at the firing time. Accordingly, thecontrol circuit 215 may be configured to disable the overcurrentprotection circuit 230 while the controllably conductive device 210 isnon-conductive to prevent the overcurrent protection circuit 230 fromcontrolling the gate voltages V_(G1), V_(G2) of the FETs Q212, Q214until after the controllably conductive device is rendered conductive atthe firing time each half-cycle (e.g., until after the first delayperiod T_(DELAY1)).

What is claimed is:
 1. A load control device for controlling powerdelivered from an alternating-current (AC) power source to an electricalload, the load control device comprising: a controllably conductivedevice adapted to be coupled between the AC power source and theelectrical load for conducting a load current through the electricalload to control the power delivered to the electrical load; a controlcircuit configured to control the controllably conductive device to benon-conductive at the beginning of each half-cycle of the AC powersource and to render the controllably conductive device conductive at afiring time during each half-cycle; and an overcurrent protectioncircuit coupled to the controllably conductive device and configured torender the controllably conductive device non-conductive in the event ofan overcurrent condition in the controllably conductive device; whereinthe control circuit is configured to generate an enable control signalfor disabling the overcurrent protection circuit when the controllablyconductive device is non-conductive and enabling the overcurrentprotection circuit after the firing time when the controllablyconductive device is rendered conductive during each half-cycle.
 2. Theload control device of claim 1, wherein the controllably conductivedevice comprises first and second field-effect transistors (FETs)electrically coupled in anti-series connection.
 3. The load controldevice of claim 2, wherein the control circuit is configured to controlthe FETs using a forward phase-control dimming technique.
 4. The loadcontrol device of claim 3, wherein the control circuit is configured todisable the overcurrent protection circuit when at least one of the FETsis controlled to be non-conductive to render the controllably conductivedevice non-conductive.
 5. The load control device of claim 4, whereinthe control circuit is configured to enable the overcurrent protectioncircuit after a first delay period from the firing time when at leastone of the FETs is controlled to be conductive to render thecontrollably conductive device conductive during each half-cycle.
 6. Theload control device of claim 5, wherein control circuit is configured todisable the overcurrent protection circuit after a second delay periodfrom a time at which the at least one of the FET is controlled to benon-conductive to render the controllably conductive devicenon-conductive during each half-cycle.
 7. The load control device ofclaim 2, wherein the control circuit is configured to generate first andsecond drive signals for controlling the first and second FETs,respectively.
 8. The load control device of claim 7, further comprising:a first gate drive circuit coupled to receive the first drive signalfrom the control circuit and generate a first gate voltage at the gateof the first FET; and a second gate drive circuit coupled to receive thesecond drive signal from the control circuit and generate a second gatevoltage at the gate of the second FET.
 9. The load control device ofclaim 8, wherein the overcurrent protection circuit is coupled to gatesof the first and second FETs to control the magnitudes of the respectivegate voltages to approximately zero volts to render the FETsnon-conductive in the event of the overcurrent condition.
 10. The loadcontrol device of claim 2, wherein the overcurrent protection circuit iscoupled across the controllably conductive device and is configured todetect the over current condition in response to a respective onresistance of each of the FETs.
 11. The load control device of claim 10,wherein a magnitude of the voltage developed across the FETs isrepresentative of a magnitude of the load current.
 12. The load controldevice of claim 10, wherein a magnitude of the voltage developed acrossthe series combination of the FETs is dependent upon a respective onresistance of each of the FETs.
 13. The load control device of claim 1,wherein the controllably conductive device comprises a field-effecttransistor (FET) in a full-wave rectifier bridge.
 14. The load controldevice of claim 13, wherein the control circuit is configured to controlthe FET using a forward phase-control dimming technique.
 15. The loadcontrol device of claim 14, wherein the control circuit is configured todisable the overcurrent protection circuit when the FET is controlled tobe non-conductive.
 16. The load control device of claim 13, wherein thecontrol circuit is configured to generate a single drive signal forproducing a single gate voltage at the gate of the FET in the bridge torender the FET conductive.
 17. The load control device of claim 16,wherein the overcurrent protection circuit is coupled to gate of the FETto control the magnitude of the gate voltage to approximately zero voltsto render the FET non-conductive in the event of the overcurrentcondition.
 18. The load control device of claim 13, wherein theovercurrent protection circuit is coupled across the FET and isconfigured to detect the overcurrent condition in response to a voltagedeveloped across the FET.
 19. The load control device of claim 18,wherein a magnitude of the voltage developed across the FETs isrepresentative of a magnitude of the load current.
 20. The load controldevice of claim 1, wherein the overcurrent protection circuit isconfigured to render the controllably conductive device non-conductiveif the magnitude of the load current exceeds an overcurrent threshold.